This invention relates in general to the field of synchronous switching operations in power lines. In particular, the present invention relates to a method and an apparatus for more accurately calculating the time of arcing contacts separation of a high voltage switchgear.
As it is well known, power systems for transmitting and distributing electricity from power sources to various loads and users are equipped with several types of protecting switchgear, such as high-voltage circuit breakers. Such switchgear are typically adapted for intervening under determined operating conditions so as to ensure a proper functioning of an associated power line and of loads/users connected therewith.
Voltage and current transients generated during switching of high-voltage circuit breakers are of increasing concern for the electric utility industry. These concerns include both power quality issues for voltage-sensitive customer loads, and excessive stresses on power system equipment. Some proposed solutions for reducing switching transients include circuit breaker pre-insertion devices, such as resistors or inductors, and fixed devices such as arresters and current limiting reactors.
A solution finding increasing popularity is the so-called synchronous switching method, sometimes also referred to as the point-on-wave switching. Synchronous switching is performed by dedicated electronic devices which—upon receiving a close or a trip command—delay the energization of the circuit breaker control coils by a few milliseconds. In this way, the current inception in the case of a close command, or the contact separation in the case of an opening or trip command, is expected to coincide with a certain point on the AC wave which is known to reduce switching transients. For synchronous closing, this point is often the voltage zero crossing. Applications where it is beneficial to close the contacts on or near the voltage zero crossing include the energizing of capacitor banks and energizing of unloaded lines or cables. Synchronous opening can be employed for shunt reactors de-energizing as an example.
The risk of re-ignitions, which are often accompanied by potentially harmful transients, is greatly reduced by moving the time of arcing contacts separation to a point on the current wave which is from one to two milliseconds after a zero crossing. This ensures that the contacts have at least one half-cycle minus two milliseconds to reach a safe dielectric distance before the occurrence of the next zero crossing.
To ensure that the targeted point on-wave coincides with the separation of the arcing contacts, the opening time of each pole has to be determined very accurately. Various equations and models are employed to calculate changes of the circuit breaker opening time depending on mechanism temperatures, control voltages and other external parameters. Gradual changes of the circuit breaker opening time that are not caused by external parameters can be captured using a method called feedback adaptation. As part of the feedback adaptation, the time difference between the arcing contacts separation and the targeted point-on-wave is used to calculate a so-called adaptation adjustment:tadapt.adj.,k+1=tadapt.adj,k·(1−G)+G·(ttarget−tcont.sep.)where tadapt.adj′k is the adaptation adjustment calculated from the kth synchronous circuit breaker operation, G is a weight factor between 0 and 1.0, ttarget is the targeted point-on-wave, and tcont.sep is the time of contact separation.
The adaptation adjustment is used to calculate the opening time:topening=tstd−tadapt.adj,k+f(Tmech,Vcontrol, . . . )where topening is the newly calculated opening time, tstd is the standard opening time, Tmech is the mechanism temperature, Vcontrol is the substation control voltage and f is a function yielding the change of the opening time depending on external parameters such as temperature, control voltage etc.
Under high-voltage conditions, the time of arcing contacts separation cannot be determined from the time of the circuit breaker current interruption or any other criteria based on circuit breaker current measurement. This is because the circuit breaker current continues to flow at least until the next zero crossing following the arcing contacts separation. Therefore, the time of opening of auxiliary contacts coupled to the arcing contacts of the circuit breaker is used in lieu of the separation time of the arcing contact.
When using the time of auxiliary contacts opening in lieu of the time of arcing contacts separation, aligning the opening of the auxiliary contacts with the arcing contacts of the circuit breaker is difficult and a time difference of a few milliseconds may remain.
Potential inaccuracies in obtaining correct feedback signals may cause inaccuracies in calculating the circuit breaker opening time, which in turn would cause inaccurate synchronous opening operations.
Therefore, it would be desirable to obtain a more accurate calculation of the separation time of the arcing contacts of a high-voltage switchgear, such as a high-voltage circuit breaker. This solution is provided by the method and apparatus of the present invention.